There have been numerous processes previously proposed for the manufacture of alkaloid carbonate, various sulfates, and the desulfurization of flue gases. One of the primary difficulties with the known procedures for manufacturing, for example, sodium bicarbonate and ammonium sulfate is the fact that a pure product is difficult to obtain when one employs the methods previously set forth in the art.
Typical of the previously proposed methods in this art is U.S. Pat. No. 3,846,535, issued Nov. 5,1974 to Fonseca. The Fonseca reference teaches a method for absorbing sulfur oxides from gaseous mixtures containing sulfur oxides. A sodium bicarbonate or potassium bicarbonate absorbent is employed to achieve this result and the absorbent is regenerated. The Fonseca reference delineates how calcium sulfate may be produced for wallboard, landfill and other such products, but fails to teach a method for producing either pure sodium bicarbonate or ammonium sulfate in purity levels suitable for commercial sale and further for flue gas desulfurization. The Fonseca disclosure indicates that only 67% of the theoretical amount of sodium bicarbonate obtained has an 87% purity therefore leaving a continuous process imbalance without external sodium bicarbonate supply.
A further limitation in the Fonseca reference is that there is no teaching concerning a source of ammonium ions to enhance the effectiveness of the process and result in desirable production of ammonium sulfate and sodium bicarbonate.
Demonstrative of the imprecise teachings in the reference regarding the procedure to achieve the most desirable process and quality products is the passage in column 3 beginning at line 52, wherein it is indicated:
"The aqueous ammonium sulfur oxide mixture produced by the precipitation of sodium bicarbonate can optionally be oxidized and recovered as ammonium sulfate. The ammonium sulfate is useful as a fertilizer and for numerous other industrial purposes."
If this were practiced, there would be a deleterious effect to the overall directive of producing sodium bicarbonate and ammonium sulfate.
Further prior art in this file includes that taught in Canadian Patent No. 543,107, issued Jul. 2, 1957, to Downes. The reference teaches a method of separating polybasic acids from their aqueous solutions and the recovery of ammonium sulfate from aqueous solutions. The disclosure indicates that the treatment of sodium sulfate for the production of sodium bicarbonate and ammonium sulfate may be achieved by exposing the aqueous solution of sulfate to ammonia and carbon dioxide. The result is the precipitation of sodium bicarbonate. Although the Downes method is useful to recover the sodium bicarbonate, there is no teaching in the disclosure concerning how an uncontaminated product of sodium bicarbonate and ammonium sulfate, since these are reciprocal salt pairs capable of the formation of a double salt which can be produced by following the method. In addition, the method as set forth in this reference would appear to be susceptible to the formation of hydrates one being known as Glauber salt when using these salt pairs.
Stiers, in U.S. Pat. No. 3,493,329, issued Feb. 3, 1970, teaches a method of making sodium carbonate. The Stiers method is a co-precipitation method and cannot provide for selective precipitation of desired products since the salts are reciprocal salts and form a double salt. In the Stiers method, the desire is to remove the sulfate anion to use it for the transportation of sodium cations from sodium chloride to the bicarbonating process as sodium sulfate. In addition to the above, the Stiers process involves the continuous recycling of the mother liquor which requires that the ammonium sulfate in the liquor be continuously removed or reduced from the process stream. If the ammonium sulfate reaches a saturation point in the bicarbonating stage, ammonium sulfate will co-precipitate with the sodium sulfate in the form of a double salt compound or two inseparable salts.
Stiers demonstrates a process to generate two salts and double salts rather than a pure single salt, the lafter being much more desirable from a commercial point of view.
Canadian Application Serial Number 2,032,627, offers an innovative technique to produce the desirable pure products. This method employed a number of evaporative and cooling techniques to alter the solubility of sodium sulfate and ammonium sulfate in solution and selectively precipitate the desired pure components. Lab bench scale batch testing of this method demonstrated effective results, however, continuous pilot scale testing clearly identified undesirable limitations to the process as specified. More specifically, the process is difficult to operate in a consistent and continuous mode and as such is highly susceptible to ammonia sulfate contamination with sodium sulfate, resulting in a commercially undesirable double salt product.
In greater detail of the teachings of the Canadian application, it is taught that brine remaining after screening sodium sulfate has a temperature of 95.degree. C. to 60.degree. C., the solubility of ammonium sulfate is decreased while the solubility of sodium sulfate increases. The result is that more ammonium sulfate precipitates while keeping sodium sulfate in solution.
By following the teachings, the mixed solution is supersaturated with sodium sulfate due to evaporation at 95.degree. C. and specifically results in the production of double salt when the ammonium sulfate crystallization step of 60.degree. C. is attempted as a continuous process.
In view of what has been previously proposed in the art, it is clear that a need exists for a process of recovering sodium carbonate compounds and the formation ammonium sulfate from a source of sulfate which overcomes the limitations regarding purity, precipitation, selectivity and other such limitations and reduces sulfur emissions from industrial facilities. The present invention is directed to circumventing the previously encountered difficulties of reciprocating salt pairs and employing the improved process for flue gas desulfurization and inorganic value recovery.